In light of terrorist bomb explosion incidents, illicit drug smuggling incidents, and so on, that have happened frequently in recent years, inspections of suspicious substances are conducted by using metal detection devices and X-ray inspection devices, and so on at traffic terminals such as airports, public facilities, and so on. However, it is difficult to detect explosives and illicit drugs put in plastic containers. Thus inspectors depend on drug sniffing dogs and so on under present circumstances; therefore it is difficult to say that security measures are sufficiently taken.
In view of such circumstances, use of a detection devices using nuclear quadrupole resonance (hereinafter referred to as “NQR detection device”) has been proposed in recent years (see Patent Document 1, for example).
In NQR detection devices, the inspection object is irradiated with a high-frequency pulse from a transmitting coil as a radio wave having a frequency specific to the inspection object, i.e., a resonance frequency, the nuclear quadrupoles of the inspection object are excited by the radio wave, a receiving coil receives a nuclear quadrupole signal (hereinafter also referred to as “NQR signal”) induced after the irradiation of the radio wave, the receiving coil amplifies the NQR signal, and a substance(s) included in the inspection object is detected, analyzed, and identified. As a result, an explosive, an illicit drug, or the like put in a plastic container can be detected.
In conventional NQR detection devices, however, inspection has been done with an inspection object placed within the transmitting/receiving solenoidal coil of each detection device.
Frequency of the radio wave for irradiation is about 500 kHz to 4 MHz, and external noises are high in level at such a radio frequency range. As a result, the external noises sometimes affect the inspection signal. Due to this reason, there was a need to take measures against the noises at the time of inspection. In conventional NQR detection devices, there was a need to provide a electromagnetic shield outside the transmitting/receiving coil to completely suppress environmental noises.
FIG. 9 shows a hand luggage inspection detection as an example of conventional NQR detection devices. In FIG. 9, reference numeral 21 denotes an antenna coil. Reference numerals 22 and 23 each denote an electromagnetic shield having a tunnel structure, and the antenna coil 21 is completely covered with the electromagnetic shield 22 and 23. And further, at a bottom portion of the electromagnetic shield 23, a belt conveyor (not shown) is provided. Reference numeral 24 denotes a personal computer (PC). The above components are placed on an installation stage in which an electronic circuit is housed.
A bag 25, in which an inspection object 26 is put, is conveyed into the tunnel-type electromagnetic shield 23, and passes through the antenna coil 21 shielded from external noises by the electromagnetic shields 22 and 23. At that time, the antenna coil 21 irradiates the bag 25 which the inspection object 26 is put in with a radio wave having a predetermined wavelength, following which a NQR signal is received. The received NQR signal is processed by the PC 24. When an explosive or an illicit drug has been detected as a result of the processing, an image of the substance is displayed on the screen of the PC 24. And further, a warning lamp (not shown) may light up as necessary.